In soft-materials friction, the roughness of rigid countersurfaces plays a significant role, since it affects the amplitude and frequency of internal strain. This study analyzes steady sliding friction between a planar Kelvin-Voigt viscoelastic foundation and a nominally flat rigid plate with periodic asperities under two constraint conditions imposed on the rigid plate: position- and force-controlled. Although previous studies using a single-asperity model have revealed that only the latter condition exhibits bell-shaped velocity-dependent friction, numerical simulations of the present multiple-asperity model show bell-shaped friction even under the former condition. The model’s simplicity analytically yields the complete set of dimensionless numbers and the asymptotic solutions. Conclusively, we find two crucial roles of surface roughness: the stationary contact state and the deformation history. The former explains why we cannot obtain a single master curve for the present model. The latter provides a different mechanism for the bell-shaped friction than the single-asperity model.
Rolling contact fatigue can be enhanced by corrosion affecting the running surfaces in rolling bearings. Corrosion can trigger different phenomena affecting rolling contact fatigue performance. The present author has published a modelling methodology based on spall propagation from initial corrosion marks at the surface. The severity of the corrosion is assessed by whether the marks will produce a propagating spall at certain load. However, corrosion can weaken the overall steel strength to fatigue and in poor lubrication conditions surface-initiated fatigue can also develop at the bearing rolling surfaces, enhanced by the corrosion effects. In this paper the methodology presented earlier is improved by adding steel overall corrosion-weakening effects and the possibility to enhance surface-initiated fatigue and spall propagation from corrosion effects. In rolling bearings, some surface-initiated related bearing failures might have their source in standstill or pitting corrosion. Standstill corrosion can be an important issue in applications with constant starts and stops, like transportation, energy, pulp and paper and others where changes in temperature and moisture condensation is likely to happen. Pitting corrosion can also be produced by the continuous exposure of steel to corrosive environments, like some solvents, water and refrigerant chemicals. If the bearing runs again, surface damage can be triggered by rolling contact fatigue or surface-initiated fatigue and propagate, eventually producing a large spall and a bearing failure. The present paper explores both mechanisms surface-initiated fatigue and spall propagation as enhanced by overall corrosion effects in fatigue.
The growing demand for greater efficiency in electric vehicles requires e-axle gears to operate at higher rotational speeds while maintaining durability. However, high-speed operation promotes lubricant scattering and viscosity reduction, creating severe lubrication conditions that accelerate micropitting fatigue. Although numerous studies have investigated micropitting under low-speed, dip-feed lubrication conditions, only limited systematic information exists about micropitting behavior under high-speed operation with forced-feed lubrication. In this study, a three-roller-type micropitting tester was employed to clarify how the lubrication supply methods and sliding conditions affect micropitting damage under high-speed sliding conditions. First, dip-feed and forced-feed lubrication were compared at an entrainment speed of 3 m/s, Hertzian contact pressures of 3.0 and 3.5 GPa, and slide-roll ratios (SRR) of −0.5%, −2.0%, and −5.0%. Subsequently, the entrainment speed was increased to 6 m/s, and the effect of the lubricant flow rate under forced-feed lubrication was evaluated by varying the flow rate from 0.5 to 15 mL/s at the same SRR levels, with dip-feed lubrication included as a reference. Micropitting damage was quantified by the damaged area percentage. Cross-sections of the sliding surfaces were observed using scanning electron microscopy to evaluate the crack length and propagation angle. The results demonstrated that forced-feed lubrication significantly reduced micropitting compared with dip-feed lubrication. Higher lubricant flow rates initially decreased micropitting, but no further improvement occurred above 10 mL/s. These findings indicate that the lubrication stability under high-speed conditions, ensured by sufficient oil replenishment and effective heat dissipation, plays a crucial role in suppressing micropitting in e-axle gear applications.
This paper presents a 6 DOF nonlinear vibration model to analyse the vibration responses caused by a deep groove ball bearing having localized defect of 2 mm circular diameter and surface waviness (waviness amplitude = 50 μm and waviness orders = 9 and 20) on its races. The model includes the mass of the shaft, ball masses, housing mass, damping lubrication, centrifugal forces, and the slip condition. The equations of motions for this model have been derived using Newton’s method. The Runge-Kutta method of the ode45 solver analyses the vibrational signals of bearings with various defects. Test bearings with localized defects and inner and outer race waviness orders 9, and 20 for theoretical analysis and waviness order 9 for experimental analysis were considered. The two races of the bearing were manually lapped to introduce artificial waviness. A coordinate measurement machine (CMM) is used to scan the artificially generated race waviness profile. The number of waves and their waviness amplitude were then determined by transferring the photos to CAD modeling software. Results of vibration responses obtained from simulation and experiments are compared and found to be in good agreement. Both theoretical and experimental results are used to estimate the cage frequency, shaft rotational frequency, and wave passage frequency (WVF or BPFI), as well as their sidebands at WVF ± fs = (nb*(fs−fc) ± fs). For waviness order 20, additional frequency peaks at sideband harmonics and sideband frequencies are determined. The results from analysis and experiments show peaks at the outer race defect frequency BPFO = nb*fc and its harmonics. Furthermore, peaks at ball slip frequency (fslip) and its corresponding ± fslip sidebands are obtained in both theoretical and experimental spectra, suggesting ball slip over the races.
In the aerospace industry, the increasing use of robot-assisted machining has led to a growing demand for drills with low cutting resistance. In this study, we investigated the cutting performance of tantalum-containing tetrahedral amorphous carbon (ta-C:Ta) coatings, which have been reported to exhibit lower friction coefficients than conventional ta-C under atmospheric friction tests. ta-C:Ta coatings with different Ta contents were deposited using an ion beam assisted filtered arc deposition (IBA-FAD) system by varying the Ta sputtering current. Their properties were characterized by nanoindentation, Raman spectroscopy, XPS, and wettability measurements. The results revealed a trade-off relationship between Ta content and both hardness and the ID/IG ratio. In addition, increasing Ta content promoted the formation of a Ta oxide surface layer, resulting in reduced surface free energy. Drilling tests on aerospace-grade PAN-based thermoset CFRP showed that thrust force after five holes decreased with increasing Ta content, with ta-C:Ta8.0 exhibiting the lowest thrust force. However, ta-C:Ta8.0 also showed the largest flank wear width among the deposited coatings, indicating inferior wear resistance. In contrast, ta-C:Ta3.5 achieved reduced thrust force while maintaining wear resistance comparable to that of conventional ta-C. These findings suggest that a Ta content of approximately 3.5% is optimal for balancing low friction and wear resistance.
Scraped textures have been applied to the slide guideways of machine tools to create fluid film lubrication. Recently, it was revealed that using lubricating oil with an acid phosphate ester additive can change solid friction in the mixed and boundary lubrication regimes to fluid friction. In a previous study, the friction coefficient and bearing gap of the guideways were evaluated using a taper-land-taper bearing (TLTB) model and a modified Reynolds equation that accounted for the effects of the high-viscosity surface layer caused by the additive oil. Using this verified theory, the present study clarifies the general bearing characteristics of TLTB-textured guideway in the high- and low-velocity regions for taper angle ranging from 0.001 to 0.006 rad. In the high-velocity region ranging from 0.125 to 2 m/s, the load capacity can be close to the maximum load-capacity condition when the taper angle is between 0.003 and 0.006 rad. The effects of the taper angle, bearing length, and load pressure on the bearing gap and stiffness were discussed. In the low-velocity region, a design method is proposed for starting and stopping within the velocity region where friction coefficient is proportional to velocity, expecting to suppress the stick-slip vibration.
Oxidation of base oils leads to the formation of insoluble deposits, such as varnish and sludge, which can cause operational problems including control valve malfunction. Consequently, reliable condition monitoring of lubricating oils is of critical importance. This study aims to clarify the influence of oxidation products on the relative dielectric constant measured using a multi-functional fluid sensor (MFS). Mineral oil–based base oil samples were subjected to stepwise oxidation using a Rotating Pressure Vessel Oxidation Test (RPVOT). Oxidation products were selectively extracted from the oxidized oils by solid-phase extraction and subsequently added stepwise to fresh mineral oil–based base oil to prepare model samples with controlled oxidation product contents. The prepared oils were evaluated using the MFS and a commercially available fluid property sensor (FPS). The results showed that the relative dielectric constant of the mineral oil–based base oil increased with oxidation progression, and a clear correlation was observed between the relative dielectric constant and the total acid number. Furthermore, the addition of extracted oxidation products was identified as a contributing factor to the increase in the relative dielectric constant. Comparison with the FPS demonstrated that the MFS could detect a correlation between the relative dielectric constant and concentration of oxidation products with higher sensitivity, enabling detection from the early stages of oxidation.
This study examines the impact of standard laser surface texturing on the tribological behavior of a conformal journal bearing with a large radial clearance. A common dimple pattern was applied to the shaft, and friction and lubricant film formation were evaluated across a wide range of speeds and loads using advanced optical diagnostics. Contrary to expectations from prior studies on non-conformal contacts, the textured surface consistently exhibited higher friction compared to the smooth reference. The strongest effects were observed in the mixed lubrication regime, where friction increased by up to 100%, and the transition to full-film lubrication was significantly delayed. High-resolution interferometry and fluorescence imaging revealed that dimples acted as lubricant sinks, locally starving the film; no cavitation was observed within or downstream of dimples. Additionally, the steep dimple geometry inhibited elastic deformation, contributing to persistent film thinning. These findings suggest that standard texturing strategies may not be suitable for conformal contacts with transitional lubrication behavior. The study highlights the importance of tailoring texture parameters to specific operating conditions and demonstrates the value of optical diagnostics in uncovering failure mechanisms in textured bearing systems.
Analysis of transient elastohydrodynamic lubrication for wavy surface contact exposed to oscillatory entrainment velocity with a Newtonian and a non-Newtonian lubricant under various surface waviness parameters of the point contact problem are presented in this paper. A sinusoidal wave characterized by two parameters of wavelength and amplitude is utilized to depict a statistical micro-contact model. To investigate the impact of lubricant rheology for Newtonian and non-Newtonian lubricants through the Ree-Eyring model, the Newton-Raphson approach along with the Gauss-Seidel iterative technique is employed to address the time-dependent Reynolds equation alongside surface deformation and statistical asperity models. The numerical results showed that the trend of the variation in central film thickness over time for non-Newtonian lubricant resembles that of Newtonian lubricant; however, a decrease in film thickness and a pressure spike were observed, unlike what is seen with Newtonian lubricants. It is evident that as the roughness amplitude rises, the distortion in the central film thickness caused by frequency diminishes. Furthermore, the findings indicated that, the behavior of transient elastohydrodynamic lubrication is really affected by the nonlinear interaction of surface waviness and non-Newtonian lubricant rheology, which lowers the pressure spike and film thickness.
In this study, the seizure resistance improvement mechanisms of nitrocarburizing, sulphonitriding, and oxynitriding were investigated to obtain design guidelines for further improving seizure resistance surface-treated steel. Roller-on-disk friction tests were conducted under high-temperature and high-contact-pressure conditions, simulating the sliding environment of high-speed gears in electric vehicles. The seizure resistance was evaluated by calculating the frictional heat generation rate per unit area and per unit time from the measured friction coefficient and contact pressure. The results indicate that the friction coefficient of sulphonitriding steel was the lowest, at 0.77 times that of carburizing steel, and that the contact pressure of sulphonitriding steel was also the smallest, at 0.77 times that of carburizing steel. As a result, the frictional heat generation rate of sulphonitriding steel was minimized to 0.73 times that of carburizing steel. These results suggest that sulphonitriding steel exhibits the highest seizure resistance among the surface-treated steels investigated.
To extend the driving range of electric vehicles (EVs), downsized e-axle systems require gears operating at high rotational speeds and sliding velocities. Under such severe rolling–sliding conditions, carburized steels are prone to fatigue wear, particularly pitting, due to friction-induced thermal softening. Nitrocarburization is a promising alternative surface treatment, yet its fatigue wear mechanism under high-sliding conditions remains unclear. Here, fatigue wear behaviors of carburized and nitrocarburized steels were systematically compared through rolling–sliding contact tests under controlled contact pressures and slide-to-roll ratios (SRR). Surface damage evolution was quantified using surface profilometry, while subsurface damage and crack propagation were examined by cross-sectional scanning electron microscopy. The two steels exhibited fundamentally different damage trends. For carburized steel, surface damage generally tended to increase with SRR, consistent with rolling contact fatigue theory, where sliding accelerates crack initiation and propagation. In contrast, nitrocarburized steel showed reduced surface damage at higher SRR. Cross-sectional observations revealed that fatigue cracks propagated deeply into the substrate in carburized steel, whereas crack growth in nitrocarburized steel was consistently arrested within the surface compound layer. Nanoindentation confirmed that this compound layer was significantly softer (≈3.6 GPa) than the carburized surface (≈10.1 GPa). These results indicate that the soft compound layer acts as a sacrificial running-in layer under sliding, promoting mild wear that smoothens the surface, reduces stress concentration, and suppresses catastrophic crack propagation. Consequently, nitrocarburization alters fatigue wear mechanisms under high-SRR rolling–sliding contact, shifting damage evolution from deep pitting to localized material detachment and providing improved fatigue resistance for high-speed EV gear applications.
Friction fade‑out (FFO) is a remarkable superlubricity in which friction coefficients approach 10⁻⁴ when a catalytic ZrO₂ pin slides against a hydrogenated DLC film in an H₂ environment containing ppm‑level ethanol vapor. This study developed a flywheel‑type tribotester to investigate FFO in pivot bearings operating at high rotational speeds under circumferential line‑contact conditions. The test bearing consisted of a 7.93‑mm ZrO₂ ball and a hydrogenated DLC‑coated SUJ2 cup. Experiments were conducted at 100–770 rpm (sliding speeds: 21–160 mm/s) under a maximum load of 73.5 N (Pmax = 0.22 GPa) in an H₂ environment with ethanol vapor. After the run‑in process, FFO was established; increasing rotational speed slightly increased the friction coefficient to 10⁻³, whereas reducing speed restored the FFO state. This reversible behavior indicates that FFO is highly sensitive to the spatial overlap of tribofilms formed on the ZrO₂ ball and the DLC‑coated cup. The appearance of FFO at low Pmax suggests that FFO is not governed solely by contact pressure but is strongly influenced by sliding speed. Post‑test observations confirmed uniform tribofilm formation and negligible wear. These findings demonstrate that FFO can be sustained under high‑speed rotation even at low line‑contact pressures, highlighting its potential for oil‑free, long‑life bearing systems.
Wheel bearings are essential components that ensure smooth rotation during both straight-line driving and cornering. A catastrophic failure of wheel bearings can lead to serious vehicle damage or even personal injury. Therefore, wheel bearings must be designed with a high level of reliability and robustness. Due to the increasing demand for quietness with the shift to Electric Vehicles, abnormal noise defects caused by denting have become an important problem in the market. Due to the above reasons, it is important to be able to predict the raceway indentations with precision during the design phase. In this research, developing a surrogate model to predict the actual indentations was completed. In order to predict actual indentations, four types of calculations were performed. First, FEM analysis was performed under specific loading conditions corresponding to actual tests in order to determine raceway loading. Second, FEM analysis was performed to get the numerical indentations values using raceway loading acquired from FEM analysis based on actual tests. Third, a surrogate model was established to predict the numerical indentations acquired from the FEM analysis using the Response Surface Method. Lastly, a surrogate model was established to predict the actual indentations using the Ridge regression. The surrogate model to predict the actual indentations provides the designer with the predicted indentations values with a high degree of precision.
Scuffing is becoming a common failure mode in gears and bearings. It has been shown that some polymethacrylates (PMAs) when used as additives can enhance the anti-scuffing performance of lubricants. This study has employed a step-sliding speed scuffing test to explore the ability of PMAs to prevent scuffing when a rolling/sliding contact is subjected to immediate severe conditions. It is found that the PMA concentration and properties such as sulphur content, Mw and functional group significantly affect scuffing protection. It is suggested that PMAs provide scuffing protection by forming thick and dense adsorption films that can withstand high sliding conditions.
Seizure occurs when a sudden heat generation at the real contact surface, called the flash temperature, becomes the source point, raising the surrounding temperature and progressing plastic flow. In previous experimental studies, in-situ observations of the contact surface between metals have been conducted, but the complete seizure process has still not been fully elucidated. We propose to apply the smoothed particle hydrodynamics (SPH) method to tribology, a solid friction simulation technique. This makes it possible to express mesoscale sliding surfaces that are difficult to express in terms of spatial scale using all-atom molecular dynamics and can predict frictional heat such as heat generation and heat conduction at the real contact surface, as well as mechanical properties such as large deformation and plastic flow. In this study, we used this simulation model to reproduce the mesoscale sliding surface of aluminum (Al) and titanium (Ti) by creating models with unevenness in the center. In this study, we performed a simulation of the sliding speed, frictional heat, and plastic flow for each material alone and for solid contact models of Al and Ti. As a result, heat generation from the center of the protrusions was confirmed for both metal materials, and then heat was diffused throughout the entire sliding surface. A similar trend is seen when a solid is in contact with a Ti solid, however, whereas the temperature rises overall at the sliding surface of Al, no significant heat diffusion is observed at the sliding surface of Ti, and considerable frictional heat is generated at the contact area. The results of the flash temprature and average temperature shown in this study are considered to indicate the physical properties of each material. In addition, plastic deformation was confirmed on the Al surface, suggesting that the generation of frictional heat and plastic deformation can be analyzed by sliding simulations.
Polyoxymethylene (POM) has excellent self-lubricating properties and is widely used in tribological applications. However, the mechanism of wear debris formation during polymer–polymer friction remains unclear. In this study, acrylic resin was employed as a transparent counter material to enable in-situ optical observations of the sliding interface formed by the surfaces of a POM ball and an acrylic resin disc. Pin-on-disk tests were conducted at different sliding speeds, and the friction coefficient and near-interface temperature were measured. In-situ optical observations and SEM analyses were also performed to evaluate wear debris flow and material transfer. At a sliding speed of 0.25 m/s, wear progressed relatively gradually, debris flow was confined to grooves and accompanied by localized stagnation, transfer to the acrylic side remained localized, and the debris mainly exhibited a flake-like morphology. In contrast, at 0.56 m/s, wear progressed more rapidly, and debris was generated over the entire POM surface, flowed continuously across the interface without significant accumulation or re-adhesion, and predominantly exhibited a roll-shaped morphology. A thick transfer layer covering the wear track was also formed on the acrylic side. These findings suggest that the sliding speed strongly influences debris flow and transfer, thereby governing the mechanism of wear debris formation in POM–acrylic resin sliding systems.
In recent years, there has been increasing demand in the fields of industrial equipment and automobiles for lighter and more compact units to save energy. The use of low-viscosity lubricant with high fluidity is known to reduce stirring resistance and decrease energy loss, delivering notable improvements in unit efficiency. As rolling bearings provide essential support to rotating components, the use of lightweight units incorporating slim and thin-section bearings has increased due to their smaller outer ring outside diameter at the same inner ring bore diameter. However, there are concerns that the bearing outer ring (the fixed ring) in these units will rotate in the same direction as the inner ring, a harmful phenomenon known as creep. In a previous study [1], we investigated the cause of creep in the thin fixed thrust washers of thrust needle roller bearings. Experimental evaluations and FEM analysis revealed that creep occurs in fixed thrust washers when circumferential strain on their back face exceeds a certain limit. In this study, a test method was established to assess the creep speed of fixed thrust washers using three types of lubricants with different kinematic viscosities. In addition, detailed observation of the thrust washer surface after testing and X-ray analysis techniques were used to verify the contribution of creep speed to rolling contact fatigue. Results indicate that although creep speed exhibits slight variation depending on differences in lubricant viscosity, lubricant additives represent a critical factor. Furthermore, controlling creep speed could significantly enhance the bearing life of thrust needle roller bearings under conditions involving substantial inclination of the support surface, which contrasts with conventional rolling contact fatigue theory.
